The present invention relates to a tag communication apparatus and a tag communication system capable of performing an RFID tag and wireless communication in cooperation with a different tag communication apparatus while scanning a beam of transmission radio waves.
In recent years, in order to manage the load, there has been used a technique for attaching an RFID (Radio Frequency Identification) tag to an article by performing a wireless communication between the RFID tag and a reader/writer. For example, when the reader/writer is installed at a carry-in entrance such as a dock door, data such as an ID (identification) is automatically read from the RFID tag attached to the article which is being carried, thereby improving efficiency of a distribution management.
At present, as a frequency band for an RFID tag, there are a 13.56 MHz band, a so-called UHF band approximately from 800 MHz to 960 MHz, a 2.45 GHz band, etc. A radio wave of the UHF band has directivity higher than that of 2.45 GHz band. In addition, the radio wave of the UHF band can be extended by a communication distance of about several tens centimeters to several meters more than the radio wave of the 13.56 MHz band. Therefore, the radio wave of the UHF band has been widely used in a distribution industry or the like in that a communication area is relatively larger at the time of using the radio wave of the UHF band.
In contrast, a problem arises in that too many RFID tags would be installed in the communication area because the communication area is larger at the time of using the radio wave of the UHF band. Accordingly, various methods of restricting the communication area within a specific area have been suggested.
For example, as one of the methods, a method of performing communication by use of the RFID reader/writer disclosed in Japanese Patent Publication No. 2006-20083A is exemplified. This RFID reader/writer is configured so as to scan a beam of a radio wave transmitted from a phased array antenna. When the RFID reader/writer is installed at one of the right and left sides of the dock door, it is possible to cover a communication area in which there is no region where communication is not possible.
However, scanning is not possible, for example, when an object through which a radio wave cannot pass is provided with a carrying article and the RFID tag is attached on a side opposite the side scanned by the RFID reader/writer in a state that the RFID reader/writer has been installed at only one of the right and left sides of the dock door.
Accordingly, in order to prevent such reading failure, the inventors have tried wireless communication with the RFID tag by installing two RFID reader/writers at both the right and left sides of the dock door so as to oppose each other.
These RFID reader/writers can be configured so as to scan the beam of the radio wave in the conveyance direction of the articles. With such a configuration, if the RFID reader/writers are installed in the dock door, it is possible to detect movement of the article which is being carried in, the article which is being carried out, or the article which is stopped, for example. Moreover, if the RFID reader/writers are installed at both the right and left sides of the dock door, it is possible to read the RFID tag even in a case where an object through which the radio wave cannot penetrate is provided with the article.
However, since the width of a general dock door is in the range from 3 to 5 m, a distance between antennas of the two opposed RFID reader/writers also becomes 3 to 5 m. If the two RFID reader/writers are installed in this way, there is a possibility that scan timing between the RFID reader/writers is not synchronized. That is, if both the RFID reader/writers simultaneously radiate a radio wave, tag confusion may be generated. Moreover, when the RFID reader/writers are installed close to each other, the radio wave of one of the opposed RFID reader/writers enters into the receiver of the other one of the opposed RFID reader/writers. Accordingly, each receiver may become saturated, so that a weak signal of the RFID tag may not be demodulated. In this case, the generation of the saturation can be prevented by lowering the gain of each receiver of the own RFID reader/writers. However, since the signal from the RFID tag is weak, the signal from the RFID tag may not be demodulated. In order to accurately demodulate the signal of the RFID tag, a distance between the opposed RFID reader/writers has to be 10 m or more.
Accordingly, in order to prevent the tag confusion from being generated, the two RFID reader/writers have to be synchronized with each other. Here, as a method of making the synchronization, a method of connecting the two RFID reader/writers through a cable any making the synchronization can be taken into consideration. However, if the cable is used to connect the opposed RFID reader/writers to each other by use of such a synchronization method, the cable has to be installed. In this case, not only additional cost for installing the cable is necessary, but also the installation of the cable may be difficult in the dock door.
Moreover, since the RFID reader/writers are connected to a server via a network in many cases, a method of making the synchronization by the server can be also taken into consideration. However, such a method of making the synchronization by use of the server has a problem that load of the server becomes increased in a case where the number of the RFID reader/writers becomes increased.